This invention relates to electronic computer systems and the like, and more particularly relates to improved methods and apparatus for achieving a video display having high resolution.
It is conventional to present the output from a computer as an image on the screen of a cathode ray tube or the like. The screen is actually composed of a collection of dots or "pixels," and the image is therefore produced by selecting and illuminating those pixels necessary to form the desired image. If the image sought to be presented is merely a simplistic pattern of numbers or other symbols, this may be achieved with a relatively limited number of pixels. However, if a more complex image (with a greater resolution) is desired, then a screen must be chosen which has a substantially greater number of pixels.
Although computer output data may be collected and exhibited in various ways, video-type output displays are especially favored for so-called "home" computers and "game" or amusement devices which are directed to the residential market. Furthermore, the residential market has increasingly demanded that these games and home-type computers exhibit output images having greater resolution. On the other hand, the residential market for these games and home computers is also extremely price conscious in character.
It should be understood that each pixel used to form the image is illuminated by a separate output data signal from the processing section of the computer, and that an increase in resolution requires a screen having a greater number of pixels. More particularly, since each video data signal must also be stored before being transferred to the video screen, an increase in image resolution also requires that the data storage section have a corresponding increase in the number of memory cells for receiving and holding all of these data signals.
If a different screen having an increased number of pixels is employed for the purpose of enhancing the resolution of the image displayed on the screen, this will not by itself cause a disproportionate increase in the overall cost of the system. However, the size or capacity of the memory component or circuit is a significant factor in the cost of the system, and an increase in the resolution of the image being presented effectively decreases the time interval available to effect a complete transfer of all of the data signals between the storage and the video section.
There have been many attempts and proposals for overcoming or mitigating these disadvantages. In particular, a larger storage unit may be selected to accomodate the increased number of input signals, but as hereinbefore explained, such a unit is inherently expensive, and its use in a home computer system will disproportionately increase the costs of such computer systems. The technology is available to provide specially designed memory units capable of fast access for higher data velocity, but such units are even more expensive than slower access memory units.
Alternatively, an increase in data storage capacity may be achieved by simply adding additional memory units. However, this not only increases the overall cost of the system, since each memory unit is a separate storage component this tends to increase the length of the time required to transfer video data to the pixels.
It has been proposed to mitigate part of the problem which arises when the data storage is composed of a plurality of separate random-access memory units or "chips," by interconnecting them in parallel with a shift register, whereby all of the units may be unloaded and their contents transferred to the shift register at the same time. The data in the shift register is then sequentially clocked to the pixels at the proper video data rate. Although this technique has been extremely beneficial in reducing the data transfer cycle to that corresponding to a single memory chip, it does not attack the problem of increased cost. Moreover, since the storage circuit is composed of memory units of standard design, there will inherently be more cells in the storage circuit than there are pixels on the video screen, and whenever the storage is unloaded into the video section, it is necessary to unload more cells than are actually required to produce the image.
These and other disadvantages of the prior art are overcome with the present invention, however, and novel means and methods are herewith provided for increasing the resolution of the image on the screen without a disproportionate increase in the overall cost of the computer system. Further, improved and novel means and methods are herewith provided for also reducing the time required to unload and transfer selected video data between the storage circuit and the video section of the computer system.